The present invention relates to Planar Inverted F-Antenna (PIFA), and in particular, to a single feed dual or tri ISM band PIFA of narrow width having a compact ground plane.
The world has witnessed a rapid progress in wireless communication. The emerging technology of short range radio links (such as the Bluetooth protocol or the like) and local area network system applications have caused a renewed focus on the industrial scientific medical (xe2x80x9cISMxe2x80x9d) frequency band. Conventionally, ISM band RF data communication devices use external antenna. But these devices could use internal antenna to avoid protruding external antenna. Internal antennas have several advantages such as being less prone to external damage, a reduction in overall size of the handset, and increased portability.
Among the various choices for internal antennas, the planer inverted F-antenna (xe2x80x9cPIFAxe2x80x9d) appears to have great promise. Relative to other internal antennas, the PIFA is generally lightweight, easy to adapt and integrate into a device chassis, has moderate range of bandwidth, has omni directional radiation patterns in orthogonal principal planes for vertical polarization, versatile for optimization, and multiple potential approaches for size reduction.
The PIFA also finds useful applications in diversity schemes. Its sensitivity to both the vertical and horizontal polarization is important for mobile cellular/RF data communication applications because of the absence of fixed orientation of the antenna as well as the multi path propagation conditions. All these features render the PIFA to be a good choice as an internal antenna for mobile cellular/RF data communication applications.
Regarding the single ISM band PIFA technology, the thrust of research has been on optimal performance with the miniaturization in the sizes of both the antenna and the ground plane. Recently, however, there is a gradual shift of the emphasis from the existing single ISM band operation to dual or tri ISM band operating covering the frequency ranges of 2.4-2.5, 5.15-5.35, and 5.47-5.725 GHz. This calls for the development of dual or tri ISM band antennas for applications in wireless communication. There exists a continued interest and requirement for the compact dual and/or tri ISM band PIFA for emerging applications of RF data wireless systems comprising laptop computer and other handheld electronic devices, such as, for example, PDAs, electronic games, cellular phones, etc.
Unlike the case of PIFA for cellular applications, in wireless RF data communication systems, there exist variations on the sizes of the radiating element and ground plane as well as on the choice of preferred placement of the PIFA within the device.
In the majority of single feed cellular dual band PIFAs, quasi-physical partitioning of the radiating element facilitates dual frequency operation. Conventionally, a slot (straight, inclined, or L-shaped) forms a quasi-physical partitioning of the radiating element to facilitate the desired physical partitioning of the PIFA structure. When the system requirements impose stringent restrictions on the allowable width of the radiating element or ground plane, such as, for example, widths as low as about 1 to about 3 mm, the conventional dual band PIFA design invoking hitherto proven slot technique can prove to be a difficult, if not impossible, task.
A conventional dual band PIFA 70 with a single feed is illustrated in FIGS. 13A and 13B. Dual band PIFA 70 has a radiating element 301 and a ground plane 302. An L-shaped slot 303 on the radiating element 301 creates a quasi-physical partitioning of the radiating element 301. The segment on the radiating element 301 with dimensions of length (L1) and width (W1) resonates at the lower frequency band of the multi band operation. Conventionally, dual band (2.4-2.5/5.15-5.35 GHz) PIFA 70 has operating dimensions of lengths between 19.16-18.38 mm for (L1) and between 12.07-11.58 mm for (W1). The segment on the radiating element 301 with dimensions of length (L2) and width (W2) resonates at the upper frequency band of the multi band operation. Conventionally, the partition results in typical operating dimensions between 8.93-8.59 mm for (L2) and 5.63-5.41 mm for (W2). A power feed hole 304 is located on the radiating element 301. A connector feed pin 305a, used for feeding radio frequency (RF) power to the radiating element 301, is inserted through the feedhole 304 from the bottom surface of the ground plane 302. The connector feed pin 305a is electrically insulated from the ground plane 302 where the feed pin passes through the hole in the ground plane 302. The connector feed pin 305a is electrically connected to the radiating element 301 with solder at 306a. The body of the feed connector 305b is connected to the ground plane 302 at 306bwith solder. The connector feed pin 305a is electrically insulated from the body of feed connector 305b. A through hole 307 is located on the radiating element 301. A conductive post 308 is connected to the radiating element 301 at 309a with solder. The conductive post 308 also is connected to the ground plane 302 at 309b with solder. The dual band impedance match of the radiating element 301 is determined by the diameter of the connector feed pin 305a, the diameter of the conductive shorting post 308 and the separation distance between the connector feed pin 305a and the conductive shorting post 308. The main disadvantage of the configuration of the multi band PIFA 70 is the lack of simple means of adjusting the separation of lower and upper resonant frequency bands. The change in the separation of the resonant frequency bands requires the repositioning of the slot 303. The above configuration is also associated with a constraint on the realizable bandwidth centered on the dual resonant frequencies of the PIFA 70.
Thus, it would be desirous to develop a dual or tri band PIFA antenna using a relatively compact antenna construct. In a related study and yet distinct from the proposed invention, the design of a single feed tri band PIFA or dual cellular and non cellular (GPS or ISM) applications has been reported in U.S. patent application Ser. No. 10/135,312, filed Apr. 29, 2002, of Kadambi et al., titled xe2x80x9cA Single Feed Tri Band PIFA with Parasitic Element,xe2x80x9d which is incorporated herein by reference.
This invention presents new and alternative design techniques of single feed Dual/Tri ISM band PIFA for wireless system applications. To attain the advantages of and in accordance with the purpose of the present invention, dual and/or tri ISM band PIFA antennas are provided. In particular, an antenna comprises at least a ground plane, a radiating element, a short, and a feed tab. The short provides a connection between the ground plane and the radiating element. The feed tab connected to the radiating element provides RF power and provides some frequency control. While the feed tab provides some frequency control, additional frequency control is obtained by the addition of one or more of a parasitic element, a slot, tuning stubs, and capacitive elements.